For many years the conveyor industry has been developing and manufacturing accumulator conveyors. In this type of conveyor the presence of articles on the conveyor is sensed and, if the article is moving normally, the conveyor continues to move the article forwardly. If, however, the forward movement of the article is impeded or halted, this fact is sensed and the propelling force applied to that article or to the next following article is released automatically. When the impediment to the forward motion of the lead article is removed, the conveyor is automatically returned to transport mode and forward motion of the articles is resumed. In many accumulators the forward motion of stationary articles on the same accumulator is sequentially reinstated zone by zone or article by article. In other accumulator designs the forward motion of the articles can be initiated substantially simultaneously such that the discharge from the accumulator is basically that of a slugged line. A number of devices have been developed for sensing the presence and state of movement of the articles along the conveyor. These include article contacting sensors such as raised rollers or wheels, air jets which are interferred with by the continuing presence of an article above the electric eyes and similar electric sensing means. All of these sensing devices respond to the presence of the article with the response being either immediate or in many cases delayed by any of several types of dampening means. The sensing units heretofore used have the drawback either of being very limited in the range of article weights capable of producing a response or being sensitive to environmental conditions such as moisture, dust and the like. The present invention is designed to provide a new, different and improved sensing means for controlling and automatically switching between transport and accumulation modes.